Global impact of smoke aerosols from landscape fires on climate and the Hadley circulation
Each year landscape fires across the globe emit black and organic carbon smoke particles that can last in the atmosphere for days to weeks. We characterized the climate response to these aerosols using an Earth system model. We used remote sensing observations of aerosol opti-cal depth (AOD) and simulations from the Community At-mosphere Model, version 5 (CAM5) to optimize satellite-derived smoke emissions for high biomass burning regions. Subsequent global simulations using the adjusted fire emis-sions produced AODs that were in closer agreement with sur-face and space-based measurements. We then used CAM5, which included radiative aerosol effects, to evaluate the cli-mate response to the fire-aerosol forcing. We conducted two 52 yr simulations, one with four sets of monthly cycling 1997–2009 fire emissions and one without. Fire emissions increased global mean annual AOD by 10 % (+0.02) and de-creased net all-sky surface radiation by 1 % (1.3 W m −2). Elevated AODs reduced global surface temperatures by 0.13 ± 0.01 • C. Though global precipitation declined only slightly, patterns of precipitation changed, with large reduc-tions near the Equator offset by smaller increases north and south of the intertropical convergence zone (ITCZ). A com-bination of increased tropospheric heating and reduced sur-face temperatures increased equatorial subsidence and weak-ened the Hadley circulation. As a consequence, precipita-tion decreased over tropical forests in South America, Africa and equatorial Asia. These results are consistent with the observed correlation between global temperatures and the strength of the Hadley circulation and studies linking tro-pospheric heating from black carbon aerosols with tropical expansion.